1. Field of the Invention
The present invention relates to a rotation transmitting apparatus provided with a differential mechanism coupling a first shaft and a second shaft that are freely rotatable so that the shafts rotate interlocking with each other, and a differential actuator for driving the differential mechanism.
2. Description of the Related Art
A rotation transmitting apparatus provided with a differential mechanism is commonly used as the steering apparatus in a vehicle. The rotation transmitting apparatus as the steering apparatus includes a sun gear mounted on a first shaft joined to a steering wheel, planetary gears rotating on its axis while moves around the sun gear, an internal gear meshing with the planetary gears and mounted on a second shaft that is disposed coaxially with the first shaft and away from the sun gear, and a carrier rotatably fitted and mounted to the first shaft and supporting the planetary gears. By operating the steering wheel, the second shaft is rotated via the first shaft, the sun gear, the planetary gears, the carrier and the internal gear, so as to operate a steering mechanism connected to the second shaft (for example, as disclosed in Japanese Patent Application Laid-Open No. 61-122071 (1986)).
In a steering apparatus disclosed in Japanese Patent Application Laid-Open No. 61-122071 (1986), teeth are provided on an outer side of the carrier, and a differential electric motor having a driving gear meshing with the teeth of the carrier, and a controller for controlling a drive circuit of the electric motor in response to a steering angle of the steering wheel and a speed of the vehicle are provided. The steering apparatus is structured such that the carrier is rotated by driving the electric motor so as to have the second shaft rotate faster than the first shaft.
In addition to the apparatus disclosed in Japanese Patent Application Laid-Open No. 61-122071 (1986), another steering apparatus including the differential mechanism is known where a reaction force electric motor is provided for applying a desired torque to the first shaft in response to the torque applied to the second shaft. The reaction force electric motor is driven when the steering torque applied to the first shaft is deviated from its correct level, e.g., by the differential electric motor increasing the rotational speed of the second shaft. Thereby, a desired torque is applied to the first shaft in response to the torque applied to the second shaft, thus returning the steering torque to its correct level.
The actions of the steering apparatus having the differential mechanism and the two, differential and reaction force electric motors as the actuators may include the following.
(1) A steady steering in which a gear ratio B/A, of a gear A on the first shaft side and a gear B on the second shaft side in the differential mechanism, is set to 1.0, and the second shaft is rotated at a speed 1.0 times as fast as a rotational speed of the first shaft by rotating the first shaft manually so as to perform a steady steering.
(2) A gear ratio increased steering in which the gear ratio B/A is set to be greater than 1.0, and the second shaft is rotated at a speed more than 1.0 times as fast as a rotational speed of the first shaft by rotating the first shaft manually.
(3) An active steering 1 in which the second shaft is rotated by a differential electric motor instead of rotating the first shaft manually so as to perform a steering.
(4) An active steering 2 in which the first shaft is rotated manually, and the second shaft is rotated by a differential electric motor in a reverse direction of the rotation of the first shaft.
(5) An automatic parking in which the second shaft is rotated by a differential electric motor instead of rotating the first shaft manually so as to park automatically.
It is known that the steering action of the vehicle having the rotation transmitting apparatus disclosed in Japanese Patent Application Laid-Open No. 61-122071 (1986) is conducted as resisting a reaction force from the ground applied to the (generally front) vehicle wheels and the reaction force may be varied depending on the running speed of the vehicle and the steering angle of the steering wheel. Accordingly, the differential electric motor is controlled and driven to increase the rotational speed of the second shaft compared to the first shaft during the slow speed running or parking and minimize a difference in the rotational speed between the first shaft and the second shaft during the high speed running.
In the rotation transmitting apparatus having the differential mechanism, meanwhile, the rotational speed of the carrier and the rotational speed of the electric motor are largely affected by the ratio between the torque applied to the first shaft and the torque applied to the second shaft. However, the torque ratio may hardly be set to an optimum while adversely affecting the steering feeling, encouraging the generation of noises at the differential mechanism, and increasing the power consumption of the electric motor(s), thus requiring a favorable solution.